Electrostatic discharge device and split multi rail network with symmetrical layout design technique

ABSTRACT

A symmetrical layout technique for an electrostatic discharge ESD device and a corresponding power supply network is presented. The ESD device protects an electronic circuit against an overvoltage or overcurrent and contains a first contact area to establish an electrical contact with a first supply rail, a second contact area to establish an electrical contact with a second supply rail, and a third contact area to establish an electrical contact with a third supply rail. The first and third supply rails provide a first supply voltage, and the second supply rail provides a second supply voltage. Within the ESD device, an axis of symmetry passes through the second contact area, and the first contact area and the third contact area are arranged on opposite sides with regard to the axis of symmetry. The symmetrical layout technique allows flipping the orientation of the ESD device with regard to the supply rails.

TECHNICAL FIELD

The present document relates to integrated circuits with electrostaticdischarge ESD protection. More specifically, the document relates to ESDdevices and corresponding layouts of a power supply network forprotecting internal circuit devices against overvoltages andovercurrents.

BACKGROUND

Typically, an ESD clamp circuit is coupled between two power supplyrails such as e.g. power and ground and is used to protect internalcircuit devices of an integrated circuit from voltage overstress. Theinternal circuit devices are also powered via the power supply rails andare accessible via I/O pads. Specifically, each internal circuit devicemay be accessible via a first I/O pad for inputting data signals and viaa second I/O pad for outputting data signals. For integrated circuitswith a plurality of internal circuit devices, each internal circuitdevice may be protected by at least one ESD clamp circuit. During an ESDstrike, overcurrents and/or overvoltages may occur at the I/O pads, andcorresponding discharge currents need to be discharged via the ESD clampcircuits to prevent the internal circuit devices from being damaged.

At this, the resistance of a discharge path from a first I/O pad to asecond I/O pad is an important design parameter. Such a discharge pathtypically includes one or more ESD clamp circuits and correspondingelectrical connections between the ESD clamp circuits and the I/O pads.In particular, the resistance of the discharge path should be chosen aslow as possible to avoid damaging the internal circuit device. In otherwords, a small resistance of the discharge path is desirable to avoid adischarge current through the internal circuit device which is protectedby the corresponding ESD clamp circuit(s). To this end, all ESD clampcircuits are usually connected to the two power supply rails and formparallel discharge paths in case of ESD strike events.

Moreover, as the number of internal circuit devices to be protectedincreases, the routing of electrical connections from the ESD clampcircuits to the respective 110 pads becomes more complicated. Forexample, routing design becomes challenging when the number of I/O padsexceeds e.g. 400. When designing the electrical connections through thedifferent layers of the integrated circuit, different constraints needto be taken into account. Such constraints include e.g. minimal pitchesbetween the electrical connections, the I/O pads, and the ESD clampcircuits. Moreover, the routing is further constrained by theorientation of the ESD clamp circuits with regard to the power supplyrails, and, finally, by the design of the power supply rails themselves.

SUMMARY

When designing a layout of the ESD clamp circuits and correspondingelectrical connections to the I/O pads, it is an objective to reduce theresistance of the overall discharge paths, wherein the overall dischargepaths also include the resistances of the electrical connections (i.e.their lengths) from the I/O pads to the ESD clamp circuits. Furtherobjectives include but are not limited to optimizing the usage of theavailable area of the integrated circuit, minimizing the required areaof the ESD protection network or simplifying the layout design of theESD protection network. The present document addresses the abovementioned technical problems. An electrostatic discharge ESD device forprotecting an electronic circuit (internal circuit device) against anovervoltage or an overcurrent comprises a first contact area, a secondcontact area, and a third contact area. The first contact area isconfigured to establish an electrical contact with a first supply rail.The second contact area is configured to establish an electrical contactwith a second supply rail, whereas the third contact area is configuredto establish an electrical contact with a third supply rail. An axis ofsymmetry passes through the second contact area. The first contact areaand the third contact area are arranged on opposite sides with regard tothe axis of symmetry.

During times in which no overcurrents or overvoltages occur, theelectronic circuit to be protected by the ESD device may be powered bythe first, the second and the third supply rail. For this purpose theelectronic circuit may be coupled between the first or the third supplyrail on the one hand and the second supply rail on the other hand. Aswill be discussed in more detail below, both the first and the thirdsupply rail may provide a first supply voltage (e.g. VDD or VSS) to theelectronic circuit, and the second supply rail may provide a secondsupply voltage (e.g. VSS or VDD, respectively) to the electroniccircuit.

The electronic circuit may form part of an overall integrated circuitand may be any kind of analog or digital electronic component such ase.g. a level shifter, a power converter or an analog-to-digitalconverter. For example, the electronic circuit may be a device whosefunctionality is made available to other components of the integratedcircuit via the I/O pads. In general, the overall integrated circuit maybe implemented using any kind of suitable packaging technology (e.g. awafer-level chip-scale package WLCSP or flip chip chip-scale packageFCCSP).

The contact areas may comprise any kind of conducting material and mayhave an arbitrary shape. Specifically, the contact areas may be threeseparated, non-overlapping areas possibly with different shapes. Forexample, a contact area may have a circular or rectangular shape of awell-defined size and position, or may even collapse to a single contactpoint which is defined e.g. by its position relative to the axis ofsymmetry. The relationship between the geometric properties of the threecontact areas of the ESD device are defined with the help of the axis ofsymmetry. At this, it goes without saying that the axis of symmetry is avirtual, fictive axis and is not embodied as a physical entity withinthe ESD device.

As the first contact area and the third contact area are arranged onopposite sides with regard to the axis of symmetry, the first contactarea and the third contact area are also arranged on opposite sides withregard to the second contact area. If now the first supply rail and thethird supply rail are configured to provide a first supply voltage(denoted e.g. as power or VDD), the second supply rail is configured toprovide a second supply voltage (denoted e.g. as ground or VSS) and thesecond supply rail is sandwiched between the first and the third supplyrail, it becomes possible to physically rotate the ESD device e.g. by180 degree with regard to the supply rails while still preserving thepolarity of the first and the second supply voltage and withoutgenerating a short-cut.

Within the ESD device, the first contact area and the third contact areamay be electrically connected with each other such that both contactareas have essentially the same electrical potential. In other words,both contact areas form a supply terminal for supplying the first supplyvoltage provided by both the first supply rail and the second supplyrail. In between said supply terminal and the second contact area, aswitching unit may be arranged. The switching unit may be implementedwith any suitable device, such as, for example, an active clamp, asimple metal-oxide-semiconductor field effect transistor (MOSFET), anIGBT, a MOS-gated thyristor, or other suitable power device. Theswitching unit may have a gate to which a respective driving voltage orcontrol signal may be applied to turn the switching unit on or off.

More specifically, the ESD device may comprise a switching unitconfigured to establish, in case the overvoltage or the overcurrent isdetected at one or more I/O pads, an electrical connection between thesupply terminal and the second contact area, and to isolate the supplyterminal and the second contact area whenever no overvoltage orovercurrent is detected. Alternatively, the ESD device may comprise aswitching unit configured to establish, in case the overvoltage or theovercurrent is detected at one or more I/O pads, an electricalconnection between the first contact area and the second contact area,and to isolate the latter contact areas otherwise. Alternatively oradditionally, the ESD device may comprise a switching unit configured toestablish, in case the overvoltage or the overcurrent is detected at oneor more I/O pads, an electrical connection between the third contactarea and the second contact area, and to isolate the latter contactareas otherwise. By connecting the contact areas, the describedswitching units establish discharge paths for protecting electroniccircuits which are located between the supply rails, too.

The first contact area, the second contact area and the third contactarea may be configured such that an orientation of the ESD device withregard to the supply rails is adjustable from a first orientation to asecond orientation. In the first orientation, the first contact areaestablishes an electrical contact with the first supply rail, the secondcontact area establishes an electrical contact with the second supplyrail, and the third contact area establishes an electrical contact withthe third supply rail. In the second orientation, the first contact areaestablishes an electrical contact with the third supply rail, the secondcontact area establishes an electrical contact with the second supplyrail, and the third contact area establishes an electrical contact withthe first supply rail. At this, the orientation of the ESD device may beadjustable from the first orientation to the second orientation byrotating the ESD device around an axis of rotation which isperpendicular to the axis of symmetry.

It should be mentioned that, both in the first and the secondorientation of the ESD device, the ESD device preferably shows the sameor at least a similar electrical behavior in terms of resistance andelectro migration current densities.

The ESD device may further comprise a terminal for coupling the ESDdevice with an I/O pad. A directional conducting device (such as e.g. adiode) may be arranged on a path between the first contact area or thethird contact area on the one hand and said terminal on the other hand.Alternatively, one may also say that the latter directional conductingdevice is coupled between the supply terminal (bridging the firstcontact area and the third contact area) and the second contact area.Moreover, the ESD device may further comprise another directionalconducting device arranged on a path between the second contact area andsaid terminal for coupling the ESD device with the I/O pad.

The directional conducting device may be e.g. a diode. In this document,the directional conducting device is considered as an electroniccomponent with at least two terminals that conducts primarily in onedirection. That is, the diode has low resistance to the flow of currentin a first direction, and high resistance in a second, oppositedirection. An ideal diode would exhibit zero resistance in the firstdirection and infinite resistance in the second direction. As anexample, a transistor operated as an active diode may as well serve asdiode in the context of this document.

The directional conducting devices may prevent current flow through thedirectional conducting devices in times when no ESD strike occurs andestablish a discharge path when an ESD strike happens. At this, adischarge current may flow e.g. from a first I/O pad through adirectional conducting device of a first ESD device to a supply rail,and over a closed switching unit of the first ESD device to anothersupply rail, and through a directional conducting device of a second ESDdevice to a second I/O pad, thereby bypassing the electronic circuit. Inaddition, between the supply rails, the discharge current may flow overclosed switching units of further ESD devices (such as e.g. the closedswitching unit of the second ESD device).

The ESD device may be arranged above or beneath the three supply rails.It may be assumed that the three supply rails (a) run locally parallel,(b) have a certain width and (c) are arranged with a minimum distancebetween each other. Depending on the point of view, the three supplyrails separate the space of the integrated circuit into two sub-spacesdenoted as a first side and a second side. The described ESD deviceprovides the advantage that an electrical connection for coupling theESD device with an I/O pad may depart on both sides of the supply railsdepending on the orientation of ESD device. On the one hand, if the ESDdevice is mounted in a first orientation on the supply rails, anelectrical connection may reach a first I/O pad on the first side of thesupply rails without crossing the supply rails. Said electricalconnection begins at the terminal for coupling the ESD device with theI/O pad and ends at the first I/O pad. On the other hand, if the ESDdevice is in the second orientation, an electrical connection may reacha second I/O pad on the second side of the supply rails without crossingthe supply rails. The electrical connection between the ESD device andthe I/O pad may be implemented e.g. partly on an assembly redistributionlayer RDL.

Put in a different way, the ESD device as well as a power supply networkcomprising a plurality of supply rails requires a minimum amount ofsymmetry such that the orientation of the ESD device may be turned withregard to the supply rails. To be more specific, a conventional dualpower supply rail system needs to be split into at least three supplyrails. If three supply rails are used, the polarity of the individualrails may be e.g. VSS-VDD-VSS or VDD-VSS-VDD. In general, the number ofsupply rails of the power supply network needs to be identical to thenumber contact areas within the ESD device. Furthermore, the polarity ofthe individual contact areas and the individual supply rails must match,both in the first orientation and in the second orientation.

It should be mentioned that, depending on the widths of the three supplyrails, the first contact area and the third contact area may be locatedat different distances from the axis of symmetry. In particular, if thewidths of the three supply rails are sufficiently large, said distancesmay be different as long as the contact areas are connectable with therespective supply rails, both in the first orientation and in the secondorientation. Nevertheless, a distance between the first contact area andthe axis of symmetry may be equal to a distance between the thirdcontact area and the axis of symmetry. At this, the distance may be e.g.determined between the centers (e.g. the geometric centers) or betweenthe borders of the respective contact areas. For instance, the distancebetween the first contact area and the axis of symmetry may be equal tothe distance between the first contact area and the second contact area.The other way round, the distance between the third contact area and theaxis of symmetry may be equal to the distance between the third contactarea and the second contact area.

The first contact area may be symmetrical to the third contact area withregard to the axis of symmetry. That is, the distances of both contactareas to the axis of symmetry may be identical. Further, the shapes ofthe contact areas may be mirrored with regard to the axis of symmetry.Alternatively or additionally, the first contact area and the thirdcontact area may be point symmetrical with regard to a symmetry point.In other words, both position and shape of the first contact area may bethe result of an inversion of the position and the shape of the thirdcontact area through the symmetry point (also denoted as pointreflection). The other way round, the geometry of the third contact areamay be derived from the geometry of the first contact area by inversionthrough the symmetry point, too. Such a symmetry point may e.g. lie onthe axis of symmetry or within the second contact area.

The three supply rails may extend along three straight, parallel lineswhich are parallel to the axis of symmetry of the ESD device.Specifically, the three supply rails may run in parallel only in awell-defined region of the integrated circuit. It should be noted thatalthough the supply rails are not part of the claimed ESD device, thegeometrical properties of the contact areas of the ESD device areimplicitly defined by the geometrical properties of the supply rails.

The ESD device may comprise a fourth contact area for establishing anelectrical contact with a fourth supply rail. In this scenario, the axisof symmetry does not pass through the second contact area, but issituated in between the second contact area and the fourth contact area.The first contact area and the third contact area are still arranged onopposite sides with regard to the axis of symmetry. In addition, thefirst contact area and the third contact area may be electricallycoupled within the ESD device and the fourth contact area and the secondcontact area may be electrically coupled within the ESD device.

In case the ESD device comprises four contact areas as described above,the power supply network should preferably comprise four supply rails,too. The inner two supply rails may have a first polarity (e.g. power orground) and the outer two supply rails may have a second polarity (e.g.ground or power).

A distance between the first contact area and the axis of symmetry maybe equal to a distance between the third contact area and the axis ofsymmetry, and a distance between the second contact area and the axis ofsymmetry may be equal to a distance between the fourth contact area andthe axis of symmetry. The first contact area may be symmetrical to thethird contact area with regard to the axis of symmetry, and the secondcontact area may be symmetrical to the fourth contact area with regardto the axis of symmetry. In a special case, the second contact area, thethird contact area and the fourth contact area may be arranged along anaxis perpendicular to the axis of symmetry.

Moreover, the ESD device may comprise a fifth contact area forestablishing an electrical contact with a fifth supply rail, wherein theaxis of symmetry passes through the fifth contact area. An ESD devicewith five contact areas requires five supply rails. The polarity of thefive supply rails may alternate in the direction of an axisperpendicular to the axis of symmetry, with two supply rails having afirst polarity and three supply rails having a second polarity.Alternatively, the outer two supply rails may have a first polarity andthe inner three supply rails may have a second polarity, or the outerfour supply rails may have a first polarity and the inner supply railmay have a second polarity.

In general, whenever the ESD device comprises an even number of contactareas, the axis of symmetry runs in between the inner two contact areas,whereas, whenever the ESD device comprises an odd number of contactareas, the axis of symmetry runs through a contact area in the middle.

According to another aspect, a power supply network for supplyingelectric power to at least one electronic circuit within an integratedcircuit is proposed. The power supply network comprises at least threesupply rails, the three supply rails extending at least partly alongthree straight, parallel lines within a power supply plane. An innersupply rail is sandwiched between two outer supply rails. The two outersupply rails each provide a first supply voltage, wherein the firstsupply voltage is different from a second supply voltage provided by theinner supply rail. In the context of this document, the term“sandwiched” may refer to a situation in which the respective supplyrails are e.g. immediately adjacent, and only separated by a minimumdistance required by the used technology. In an exemplary scenario, thefirst supply voltage or the second supply voltage may be ground.

The power supply network may comprise a fourth supply rail, the fourthsupply rail running parallel to the at least three supply rails. Theinner supply rail and the fourth supply rail may be adjacent andsandwiched between the two outer supply rails. The fourth supply railmay provide the second supply voltage to the electronic circuit and/orthe overall integrated circuit.

In other words, the power supply network may comprise at least foursupply rails, wherein two adjacent inner supply rails are sandwichedbetween two outer supply rails, and the two outer supply rails eachprovide the first supply voltage. The first supply voltage may bedifferent from the second supply voltage provided by each of the twoadjacent inner supply rails.

According to yet another aspect, an electrostatic discharge ESDprotection network is proposed. The ESD protection network comprises apower supply network as described above, a first ESD device as describedabove coupled to a first I/O pad via a first electrical connection, anda second ESD device as described above coupled to a second I/O pad via asecond electrical connection. The first I/O pad and the second I/O padare arranged on opposite sides with respect to the inner supply rail,and neither the first electrical connection nor the second electricalconnection crosses the three supply rails. Further the ESP protectionnetwork may comprise an electronic circuit which is protected by the ESDdevices, wherein the electronic circuit is electrically connected to thesecond supply rail and to at least one of the first supply rail and thethird supply rail. Further, the electronic circuit may be coupled withboth the first I/O pad and the second I/O pad for exchanging datasignals.

The first ESD device and the second ESD device may be identicallyconstructed, and the first ESD device may be rotated by 180 degree withregard to the second ESD device.

Moreover, the first and the third contact areas of the first ESD devicemay be connected with the outer supply rails and the second contact areaof the first ESD device may connected with the inner supply rail. Thefirst and the third contact areas of the second ESD device may beconnected with the outer supply rails and the second contact area of thesecond ESD device may be connected with the inner supply rail. The firstand the second ESD device may be either arranged above or beneath thepower supply plane.

According to yet another aspect, a method for protecting an electroniccircuit against an overvoltage or an overcurrent using an electrostaticdischarge ESD device is proposed. The proposed method comprises thesteps of establishing an electrical contact between a first contact areaof the ESD device and a first supply rail, establishing an electricalcontact between a second contact area of the ESD device and a secondsupply rail, and establishing an electrical contact between a thirdcontact area of the ESD device and a third supply rail. An axis ofsymmetry passes through the second contact area, and the first contactarea and the third contact area are arranged on opposite sides withregard to the axis of symmetry. Thus, the method comprises the step ofarranging the first contact area and the third contact area on oppositesides with regard to the axis of symmetry. A distance between the firstcontact area and the axis of symmetry may be designed to be equal to adistance between the third contact area and the axis of symmetry.

According to yet another aspect, a method for supplying electric power,using a power supply network, to at least one electronic circuit withinan integrated circuit is proposed. The power supply network comprises atleast three supply rails. The method comprises arranging the threesupply rails at least partly along three straight, parallel lines withina power supply plane. Next, the method comprises sandwiching an innersupply rail between two outer supply rails, wherein the two outer supplyrails each provide a first supply voltage. The first supply voltage ischosen to be different from a second supply voltage provided by theinner supply rail.

According to yet another aspect, another method for supplying electricpower, using a power supply network, to at least one electronic circuitwithin an integrated circuit is proposed. This time, the power supplynetwork comprises at least four supply rails. The method comprisesarranging the four supply rails at least partly along four straight,parallel lines within a power supply plane and sandwiching two adjacentinner supply rails between two outer supply rails. In doing so, the twoouter supply rails each provide a first supply voltage, the first supplyvoltage being different from a second supply voltage provided by each ofthe two adjacent inner supply rails.

It should be noted that the methods and systems including its preferredembodiments as outlined in the present document may be used stand-aloneor in combination with the other methods and systems disclosed in thisdocument. In addition, the features outlined in the context of a systemare also applicable to a corresponding method. Furthermore, all aspectsof the methods and systems outlined in the present document may bearbitrarily combined. In particular, the features of the claims may becombined with one another in an arbitrary manner.

In the present document, the term “couple”, “connect”, “coupled” or“connected” refers to elements being in electrical communication witheach other, whether directly connected e.g., via wires, or in some othermanner.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below in an exemplary manner with referenceto the accompanying drawings, wherein

FIG. 1 shows a layout of an integrated circuit with ESD devices;

FIG. 2 shows a layout of an ESD device and power supply rails;

FIG. 3 shows a schematic illustration of ESD devices and correspondingpower supply rails;

FIG. 4 shows an exemplary layout of an integrated circuit with exemplaryESD devices;

FIG. 5 shows other exemplary layouts of an integrated circuit withexemplary ESD devices; and

FIG. 6 shows yet other exemplary layouts of an integrated circuit withexemplary ESD devices.

DESCRIPTION

As outlined above, the present document relates to electrostaticdischarge ESD devices and corresponding power supply networks forprotecting internal circuit devices (electronic circuits) of anintegrated circuit against overvoltages and/or overcurrents. FIG. 1shows a layout of an integrated circuit 1 with ESD devices 10, 11, 12,13 known from the state of the art. FIG. 1 shows ten ESD devices intotal, wherein the ESD devices are arranged above or below power supplyrails 101, 102, 103, 104. The power supply rails belong to a powersupply network, and the two left power supply rails 101, 102 supply theintegrated circuit with a first supply voltage (e.g. power, VDD, or VCC)and the two right power supply rails 103, 104 supply the integratedcircuit with a second supply voltage (e.g. ground or VSS). An internalcircuit device which is powered by the supply rails 101, 102, 103, 104is not depicted in FIG. 1.

The ESD devices comprise four contact areas (not shown) for establishingelectrical contacts with the respective supply rails 101, 102, 103, 104.For this purpose, two contact areas for connecting with the left supplyrails 101, 102 are arranged in the left halves of the ESD devices, andtwo contact areas for connecting with the right supply rails 103, 104are arranged in the right halves of the ESD devices.

Electrical connections leave the ESD devices and connect each ESD devicewith a pad 100, 110, 120, 130 (also denoted as I/O pad or pad opening),which in turn connects the ESD devices with balls 121, either directlyor via further routing on an assembly redistribution layer RDL. From theupper four ESD devices and the lower four ESD devices, the electricalconnections extend to the left of the supply rails 101, 102, 103, 104.For example, a first electrical connection departing from ESD device 12reaches a ball 121 via pad opening 120. The pad opening 120 may be e.g.a hole in a passivation layer which protects the chip against touching.The pad opening 120 may connect the RDL with an aluminum layer in whichthe supply rails 101, 102, 103, 104 are implemented. As another example,a second electrical connection departing from ESD device 13 reaches asecond I/O pad 130. Departing from I/O pad 130, RDL wiring 131 of theRDL layer connects ESD device 13 with another ball (not shown) locatedfurther away from the supply rails 101, 102, 103, 104.

As can be seen in FIG. 1, all pad openings have to be positioned on theleft side of the supply rails 101, 102, 103, 104 due to the polarity ofthe supply rails. If a ball on the right side of the supply rails needsto be connected with a pad opening on the left side, the intersectingconnections 14, 15, 16, 17 which cross the supply rails need to bedesigned. Intersecting connections 14, 15, 16, 17 may be placed in alayer above or below the supply rails. In the illustrated example,intersecting connections 14, 15, 16, 17 couple ESD devices 10 and 11 viapad openings 100 and 110 with the two balls on the right side of thesupply rails 101, 102, 103, 104. As a disadvantage, additional space isrequired which separates ESD devices 10 and 11 from the remaining ESDdevices and is used for intersecting connections 14, 15, 16, 17. As afurther disadvantage, the lengths of the electrical connectionsextending to the ride side increases, resulting in an unwanted higherresistance of the respective discharge paths. As yet anotherdisadvantage of the illustrated example from the prior art, the routingof the electrical connections, in particular the routing of theelectrical connections on the RDL layer is complicated due to theuniform, parallel orientation of the ESD devices.

Obviously, with the layout of the ESD devices and supply rails 101, 102,103, 104 depicted in FIG. 1, it is not possible to turn an ESD device by180 degree to facilitate routing to balls located on the right side ofthe supply rails, since such a rotation would inevitably lead to ashort-cut of the power supply system.

FIG. 2 provides an enlarged view on a single ESD device 20 which isconnected with a pad opening 200. ESD device 20 may be e.g. one of theten ESD devices depicted in FIG. 1. The exemplary ESD device 20comprises an active clamp cell 21 which acts as a switching unit forconnecting supply rails with each other in the event of an ESD strike.More precisely, the latter switching unit is configured toconnect/disconnect supply rails 101, 102 supplying the integratedcircuit with the first supply voltage with/from supply rails 103, 104supplying the integrated circuit with the second supply voltage.Moreover, the ESD device 20 comprises a first diode 22 and a seconddiode 23. The first diode 22 is arranged on a discharge path betweensupply rails 101 and 102 and pad opening 200. The second diode 23 isarranged on a discharge path between supply rails 103 and 104 and padopening 200. Alternatively, the two diodes 22, 23 may be positionedexternally e.g. at the pad opening 200 or at the supply rails 101, 102,103, 104 and, thus, may not be part of the ESD device 20 itself. It isthe duty of the diodes 22, 23 to prevent current flow through the ESDdevice 20 in times when no ESD strike occurs and to establish adischarge path through the ESD device when an ESD strike happens.

The top schematic of FIG. 3 displays a conventional ESD protectionnetwork 31 with two supply rails 311 and 312. As already described inthe foregoing, conventional ESD devices 32, 33 may only comprise twocontact areas 321, 322 and 331, 332 which do not allow rotation of theESD devices 32, 33 with regard to the supply rails 311 and 312.Consequently, pad openings 320, 330 may only be reached on one side(here the upper side) of the supply rails without crossing the supplyrails.

The bottom schematic of FIG. 3 displays an exemplary ESD protectionnetwork 34 as proposed in the present document. The illustrated examplefor an ESD protection network 34 comprises three ESD devices 35, 36, 37which are designed symmetrically in a sense that the inner contact areas352, 362, 372 are located in between two outer contact areas 351, 353,361, 363, 371, 373. An exemplary axis of symmetry 3000 passes throughthe inner contact areas 352, 362, 372. As the two supply rails 311, 312are now split up into three supply rails 341, 342, 343, wherein theinner supply rail 342 has a different polarity than the outer supplyrails 341, 343, it becomes possible to flip ESD devices with regard tothe supply rails and reach pad openings 350, 360, 370 on different sides(i.e. the upper and the lower side) of the supply rails without crossingthe supply rails. In the displayed example arrangement, pad opening 360is located above the supply rails and pad openings 350 and 370 arelocated below the supply rails.

FIG. 4 shows a layout of an integrated circuit 4 with exemplary ESDdevices 401 to 410 in accordance with the principles presented in thisdocument. This time, the inner supply rails 102, 103 have a firstpolarity, and the outer supply rails 101, 104 have a second polarity. Inaddition, the arrangement of contact areas within the ESD devices allowflipping the ESD devices by 180 degree without generating a short-cut.In comparison to the integrated circuit 1 illustrated in FIG. 1, onlysix ESD devices 402, 403, 404, 407, 408, 409 are oriented towards theleft side of the supply rails. The remaining four ESD devices 401, 405,406, 410 are oriented to the right side to connect the ESD devices 401,405, 406, 410 with balls on the right side. The latter balls may bereached either via pad openings 45, 48, or via pad openings 46, 47.Starting from pad openings 46, 47, the balls 49, 50 may be reached usingwiring 41, 42, 43, 44 of the RDL layer.

In FIG. 4, each ESD device may comprise an active clamp for connectingsupply rails with opposite polarities with each other in the event of anESD strike. That is, an active clamp may be configured to short-cutsupply rails 101, 104 with supply rails 102, 103 in the event of an ESDstrike, and to isolate the latter supply rails from each other duringregular operation of the integrated circuit. Similarly, each ESD devicemay comprise a first directional conductor (e.g. a diode) coupling therespective pad with supply rails 101, 104. On the other hand, each ESDdevice may comprise a second directional conductor coupling therespective pad with supply rails 102, 103.

In comparison to the layout of FIG. 1 which is known from the prior art,the example layout in FIG. 4 enables improved RDL routing flexibility,shorter overall discharge paths and hence discharge paths with lowerresistance. Moreover, die size area is saved due to the smaller pitchbetween the ESD devices. Specifically, the intersecting connections 14,15, 16, 17 of FIG. 1 become dispensable and are replaced by wiring 41,42, 43, 44. The four ESD devices 404, 405, 406, and 407 in the middlemay be arranged closer together. This reduction of the area required bythe ESD devices becomes possible by using ESD devices of the same typeand simply rotating the ESD devices with regard to the supply rails.

In order to optimize usage of the available area of the integratedcircuit and in order to minimize the resistances of the discharge paths,it may be beneficial to place ESD devices and corresponding supply railsinto the core area of the integrated circuit. FIG. 5 shows two possibledesigns for arranging the supply rails and ESD devices. The left design51 shows 100 balls 513 connected with ESD devices which are arrangedalong supply rails 511 at the borders of the integrated circuit. Design51 is known from the prior art and pad openings 512 to which the ESDdevices are connected are all located on the same side of the supplyrails 511. That is, two supply rails 511 are sufficient, wherein onesupply rail provides e.g. power VDD and the other supply rail providesground VSS to the integrated circuit.

On the other hand, the right design 52 is an example based on the ideaspresented in this document. At least 3 supply rails with alternatingpolarity are arranged along a rectangle which separates the center ofthe integrated circuit from a border region. In the border region, 64balls 523 are connected through pad openings 522 with ESD devices beingarranged on the supply rails in a first orientation. In the centerregion, 36 balls 523 are connected through pad openings 522 with ESDdevices being arranged on the supply rails in a second orientation whichis reversed with regard to the first orientation. As a result, the areaoccupied by the ESD protection network comprising ESD devices and supplyrails is reduced by approximately 40% compared to the left design 51. Onaverage, the resistance of the discharge paths is reduced. Further, themaximum resistance (which is e.g. important for point-to-point ESDstrike tests) among all discharge paths is reduced.

Similarly, FIG. 6 shows another example design 60 based on the ideaspresented in this document. Again, supply rails and mounted ESD devicesare placed in the core area of the integrated circuit. In design 60, abundle of at least 3 supply rails 601 with alternating polarity isplaced within the integrated circuit in an H-shaped manner. Again, ESDdevices may be placed on the supply rails 601 in one of two possibleorientations to reach pad openings 602 on both sides of the supply rails601 without having to cross the supply rails 601. The area occupied bythe ESD protection network comprising ESD devices and supply rails isreduced by approximately 40% compared to a design 51 known from theprior art.

In both designs 52 and 60, a designer may freely select the orientationof the individual ESD devices with regard to the supply rails tooptimize the resistances (i.e. the lengths) of the discharge paths i.e.to optimize routing of the electrical connections from the ESD devicesto the respective balls. This increased flexibility may be in particularbeneficial for large integrated circuits where placing ESD devices onsupply rails at the border (e.g. in the region of the sealrings) comesto its limits.

It should be noted that the description and drawings merely illustratethe principles of the proposed methods and systems. Those skilled in theart will be able to implement various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples and embodiment outlined in the present document are principallyintended expressly to be only for explanatory purposes to help thereader in understanding the principles of the proposed methods andsystems. Furthermore, all statements herein providing principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass equivalents thereof.

What is claimed is:
 1. An electrostatic discharge ESD device configuredto protect an electronic circuit against an overvoltage or anovercurrent, the ESD device comprising a first contact area configuredto establish an electrical contact with a first supply rail, a secondcontact area configured to establish an electrical contact with a secondsupply rail, and a third contact area configured to establish anelectrical contact with a third supply rail, wherein an axis of symmetrypasses through the second contact area, and the first contact area andthe third contact area are arranged on opposite sides with regard to theaxis of symmetry, wherein the ESD device further comprises a terminalfor coupling the ESD device with an external I/O pad, and a directionalconducting device arranged on a path between the second contact area andsaid terminal.
 2. The ESD device of claim 1, wherein a distance betweenthe first contact area and the axis of symmetry equals a distancebetween the third contact area and the axis of symmetry.
 3. The ESDdevice of claim 1, wherein the first contact area is symmetrical to thethird contact area with regard to the axis of symmetry.
 4. The ESDdevice of claim 1, wherein the three supply rails extend along threestraight, parallel lines which are parallel to the axis of symmetry ofthe ESD device.
 5. The ESD device of claim 1, wherein the first contactarea and the third contact area are electrically connected with eachother within the ESD device.
 6. The ESD device of claim 1 comprising aswitching unit configured to establish, in case the overvoltage or theovercurrent is detected at one or more I/O pads, an electricalconnection between the first contact area and the second contact area,and to isolate the latter contact areas otherwise.
 7. The ESD device ofclaim 1 comprising a switching unit configured to establish, in case theovervoltage or the overcurrent is detected at one or more I/O pads, anelectrical connection between the third contact area and the secondcontact area, and to isolate the latter contact areas otherwise.
 8. TheESD device of claim 1, further comprising a terminal for coupling theESD device with an external I/O pad, and a directional conducting devicearranged on a path between the first contact area or the third contactarea on the one hand and said terminal on the other hand.
 9. The ESDdevice of claim 1, wherein the electronic circuit to be protected by theESD device is supplied with electrical power by the first, the secondand the third supply rail.
 10. The ESD device of claim 9, wherein thefirst direction is a direction which is rotated by 180 degree withregard to the second direction.
 11. The ESD device of claim 1, whereinthe first contact area, the second contact area and the third contactarea are configured such that the ESD device can be placed in a firstdirection or in a second direction with regard to the supply rails,wherein, in the first direction, the first contact area establishes anelectrical contact with the first supply rail, the second contact areaestablishes an electrical contact with the second supply rail, and thethird contact area establishes an electrical contact with the thirdsupply rail, and, in the second direction, the first contact areaestablishes an electrical contact with the third supply rail, the secondcontact area establishes an electrical contact with the second supplyrail, and the third contact area establishes an electrical contact withthe first supply rail.
 12. An electrostatic discharge ESD deviceconfigured to protect an electronic circuit against an overvoltage or anovercurrent, the ESD device comprising a first contact area configuredto establish an electrical contact with a first supply rail, a secondcontact area configured to establish an electrical contact with a secondsupply rail, a third contact area configured to establish an electricalcontact with a third supply rail, and a fourth contact area forestablishing an electrical contact with a fourth supply rail wherein anaxis of symmetry is situated in between the second contact area and thefourth contact area, and the first contact area and the third contactarea are arranged on opposite sides with regard to the axis of symmetry,the ESD device further comprising a fourth contact area for establishingan electrical contact with a fourth supply rail, wherein the axis ofsymmetry does not pass through the second contact area, but is situatedin between the second contact area and the fourth contact area, and thefirst contact area and the third contact area are still arranged onopposite sides with regard to the axis of symmetry.
 13. The ESD deviceof claim 12, wherein a distance between the first contact area and theaxis of symmetry equals a distance between the third contact area andthe axis of symmetry, and a distance between the second contact area andthe axis of symmetry equals a distance between the fourth contact areaand the axis of symmetry.
 14. The ESD device of claim 12, wherein thefirst contact area is symmetrical to the third contact area with regardto the axis of symmetry, and the second contact area is symmetrical tothe fourth contact area with regard to the axis of symmetry.
 15. The ESDdevice of claim 14, wherein the first contact area, the second contactarea, the third contact area and the fourth contact area are arrangedalong an axis perpendicular to the axis of symmetry.
 16. The ESD deviceof claim 12, wherein the fourth contact area and the second contact areaare electrically connected with each other within the ESD device. 17.The ESD device of claim 13 comprising a fifth contact area forestablishing an electrical contact with a fifth supply rail, wherein theaxis of symmetry passes through the fifth contact area.
 18. A method forprotecting an electronic circuit against an overvoltage or anovercurrent using an electrostatic discharge ESD device, the methodcomprising establishing an electrical contact between a first contactarea of the ESD device and a first supply rail, establishing anelectrical contact between a second contact area of the ESD device and asecond supply rail, and establishing an electrical contact between athird contact area of the ESD device and a third supply rail, wherein anaxis of symmetry passes through the second contact area, and the firstcontact area and the third contact area are arranged on opposite sideswith regard to the axis of symmetry; wherein the method furthercomprises coupling a terminal of the ESD device with an external I/Opad, and arranging a directional conducting device on a path between thesecond contact area and said terminal.
 19. The method according to claim18, wherein a distance between the first contact area and the axis ofsymmetry equals a distance between the third contact area and the axisof symmetry.
 20. The method of claim 18, wherein the first contact areais symmetrical to the third contact area with regard to the axis ofsymmetry.
 21. The method of claim 18, wherein the three supply railsextend along three straight, parallel lines which are parallel to theaxis of symmetry of the ESD device.
 22. The method of claim 18, whereinthe first contact area and the third contact area are electricallyconnected with each other within the ESD device.
 23. The method of claim18 comprising a switching unit to establish, in case the overvoltage orthe overcurrent is detected at one or more I/O pads, an electricalconnection between the first contact area and the second contact area,and to isolate the latter contact areas otherwise.
 24. The method ofclaim 18 comprising a switching unit to establish, in case theovervoltage or the overcurrent is detected at one or more I/O pads, anelectrical connection between the third contact area and the secondcontact area, and to isolate the latter contact areas otherwise.
 25. Themethod of claim 18, further comprising the steps of: coupling with aterminal the ESD device with an external I/O pad, and arranging adirectional conducting device on a path between the first contact areaor the third contact area on the one hand and said terminal on the otherhand.
 26. The method of claim 18, wherein the electronic circuit to beprotected by the ESD device is supplied with electrical power by thefirst, the second and the third supply rail.
 27. The method of claim 18,wherein the first contact area, the second contact area and the thirdcontact area are used so that the ESD device can be placed in a firstdirection or in a second direction with regard to the supply rails,wherein, in the first direction, the first contact area establishes anelectrical contact with the first supply rail, the second contact areaestablishes an electrical contact with the second supply rail, and thethird contact area establishes an electrical contact with the thirdsupply rail, and, in the second direction, the first contact areaestablishes an electrical contact with the third supply rail, the secondcontact area establishes an electrical contact with the second supplyrail, and the third contact area establishes an electrical contact withthe first supply rail.
 28. The method of claim 27, wherein the firstdirection is a direction which is rotated by 180 degree with regard tothe second direction.
 29. A method for protecting an electronic circuitagainst an overvoltage or an overcurrent using an electrostaticdischarge ESD device, the method comprising establishing an electricalcontact between a first contact area of the ESD device and a firstsupply rail, establishing an electrical contact between a second contactarea of the ESD device and a second supply rail, establishing anelectrical contact between a third contact area of the ESD device and athird supply rail, and establishing an electrical contact between afourth contact area of the ESD device and a fourth supply rail, whereinan axis of symmetry is situated in between the second contact area andthe fourth contact area, and the first contact area and the thirdcontact area are arranged on opposite sides with regard to the axis ofsymmetry, the method further comprising the step of: establishing with afourth contact area an electrical contact with a fourth supply rail,wherein the axis of symmetry does not pass through the second contactarea, but is situated in between the second contact area and the fourthcontact area, and the first contact area and the third contact area arestill arranged on opposite sides with regard to the axis of symmetry.30. The method of claim 29, wherein a distance between the first contactarea and the axis of symmetry equals a distance between the thirdcontact area and the axis of symmetry, and a distance between the secondcontact area and the axis of symmetry equals a distance between thefourth contact area and the axis of symmetry.
 31. The method of claim 30comprising the step of: establishing with a fifth contact area anelectrical contact with a fifth supply rail, wherein the axis ofsymmetry passes through the fifth contact area.
 32. The method of claim29, wherein the first contact area is symmetrical to the third contactarea with regard to the axis of symmetry, and the second contact area issymmetrical to the fourth contact area with regard to the axis ofsymmetry.
 33. The method of claim 32, wherein the first contact area,the second contact area, the third contact area and the fourth contactarea are arranged along an axis perpendicular to the axis of symmetry.34. The method of claim 29, wherein the fourth contact area and thesecond contact area are electrically connected with each other withinthe ESD device.